Electrical system for automatic arc welding

ABSTRACT

A process and system for welding of the tungsten inert gas (TIG) type is described. A welding torch is mounted on a mechanism which provides for motion of the torch with respect to the work (a pipe being welded) in three different directions. Movement in each direction is electronically controlled so as to obtain preset magnitudes of oscillation of the torch, both vertically and laterally with respect to the work. The vertical oscillation is provided by following the pulsating arc current in accordance with a preselected control function. In addition, the system and process provides for control of substantially all of the parameters affecting the formation of the weld in order to produce improved weldments.

This is a division of application Ser. No. 136,503 filed Apr. 22, 1971,which is a continuation of application Ser. No. 804,724 dated Oct. 29,1968, now abandoned.

The present invention relates to methods and systems for arc welding,and particularly to systems and process for arc welding wherein a pulsecurrent is supplied to form the arc.

The invention is espcially suitable for use in pulsed arc weldingsystems of the nonconsumable electrode type which are also known astungsten inert gas or TIG welding systems. While the invention isdescribed in connection with the welding of joints between sections ofpipe, it will be appreciated that features of the invention provide forthe automatic control of substantially all parameters affecting theformation of a weld and therefore the invention may be applied toaccomplish welding regardless of the type of weldment, joint design,positional relationship of torch with respect to the work or themetallurgical characteristics of the materials to be welded.

In the practice of pulsed arc welding, a pulsating welding current isapplied to the torch during the welding cycle in order to control boththe melting and solidification of the weld. The control of the waveformof the current pulses provides a measure of control of resulting weld.It has been found, however, that additional control is desirable inorder to accomplish fully automatic arc welding of the various jointdesigns, materials, and work to torch positional relationships which maybe encountered. Automatic arc welders which are available have beenlimited to applications where the work remains essentially in the sameposition with respect to the weld. While control over the pulsed arcpermits greater flexibility in weld torch positions, more flexibility isrequired in order to provide for universal automatic arc welding.

It has been found in accordance with the invention that additionalparameters which affect the weld may be provided and controlled in amanner integrated with the control of the pulse arc so as to provide apractical fully automatic arc welding machine. A welding machineembodying the invention therefore is adapted to produce precision weldsand to operate in all positions, thus making automatic welding possiblewhere only slow manual welding techniques could heretofore be used. Theparameters which have been made available for control in accordance withthe invention are an oscillatory movement of the torch with respect tothe work, both in a vertical direction (viz. height control) and in atransverse direction (viz. laterally across the weld). The oscillationsmay be synchronized with the current pulses supplied to the arc in orderto provide integrated control of the formation of the weld (viz. tocontrol melting and fusion).

Thus, it is a feature of the invention to provide controllableparameters which are effective in determining the nature of a weld andto integrate the control of such parameters so as to make the weldsrapidly, precisely and irrespective of the position of the weld withrespect to the torch. Among the parameters referred to above are currentmagnitude, current pulse, shape and rate, oscillatory movement of thetorch with respect to the work, both vertically and laterally, andlongitudinal movement of the work with respect to the torch (viz. travelof the torch along the weld).

Further features of the invention are to provide protection both of thework and the torch in the event of failures in the system such as lossof inert gas, cooling water, breakdown of electrical components and thelike.

Still further features of the invention reside in the ease with whichthe system may be set up and adjusted to accommodate various sizes ofwork.

Another feature which is afforded by the availability of control overall of the welding parameters is that it facilitates the use of lowerarc voltages, thereby reducing the possibility of burning and otherdamage to the work.

Still another feature arising out of the availability of the controlledwelding parameters is precise quantized agitation of the weld puddle soas to control the metallurgical characteristics of the weld. The controlover the weld parameters also provides control of the contour of theweld and substantially eliminates undercutting of the side walls of thewelded joint.

The principal object of the invention therefore is to provide andimproved process of and system for pulsed arc welding which is capableof producing weldments automatically.

A still further object of the invention is to provide an improvedprocess of and system for pulsed arc welding which controls parameterswhich are effective in determining the nature of a weld so as to producebetter welds faster than heretofore feasible.

A still further object of the invention is to provide improved systemsand processes for controlling in an integrated manner, all theparameters which are effective in determining the nature of a weld madeby pulsed arc welding.

A still further object of the present invention is to provide anautomatic welding machine having the features mentioned above.

Fully automatic arc welding requires that the arc be initiatedautomatically and in a manner which will prevent damage to the torch aswell as to the work. In earlier automatic arc welding machines, arcswere started by applying high frequency energy in the region between thetorch electrode and the work. The high power high frequency generatornecessary to supply the starting energy is both costly and providesundesirable side effects, such as interference with radio transmissionand reception.

It has been found in accordance with this invention that such highfrequency techniques may be eliminated and the arc initiated byautomatic means for applying the starting current to the torch in timedrelationship with the movement of the torch from a position in contactwith the work to a position away from the work.

Accordingly, it is an object of the invention to provide an arc weldingsystem having simplified means for initiating the arc which prevents anydamage, either to the torch or to the work during arc initiation.

Briefly described, a method of and system for pulsed arc weldingembodying the invention includes the step of applying pulsed weldingcurrent to provide an arc between the work and a welding torch. Themagnitude, wave shape and frequency of this current may be preset inorder to provide the desired weld formation characteristic. The torch isalso oscillated vertically with respect to the work in synchronism withthe pulsating arc current, thereby providing another parameter effectivein determining the nature of the weld. This parameter may be controlledby sensing the pulsating arc voltage and generating a control signal inresponse thereto which is adapted to move the welding torch in adirection and with an amplitude to provide the desired control over thisvertical oscillation parameter. The welding torch may also be oscillatedlaterally with respect to the weld to provide another parameter whichmay be preset, both as regards the amplitude and location of the lateraloscillatory movement of the torch with respect to the weld. The movementor travel of the torch with respect to the work lengthwise along theweld is also controlled so that the arc is initiated and all of theother parameters affecting the weld are established and are maintainedin proper time relationship over successive regions of the weld.

Mechanisms are provided for positioning and oscillating the torch aswell as for moving the entire torch assemblage with respect to the workso as to travel along the weld. These mechanisms include means foradjustment to accommodate various sizes of work in a manner not tointerfere with the oscillation of the torch, both vertically andlaterally with respect to the weld.

The system for initiating the arc utilizes the means for verticallyoscillating the torch to touch the work and further includes circuitsfor detecting the starting current and moving the torch away from thework. A control circuit is also provided for electrically preventing thetorch from leaving the work until the current supplied to the torch iswithin the range which will start the arc without causing damage to thework.

The invention itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof willbecome more readily apparent from a reading of the following descriptionin connection with the accompanying drawings in which:

FIG. 1 is a block diagram of a welding system embodying the invention;

FIG. 2 is a more detailed block diagram of the programming system shownin FIG. 1;

FIG. 3 is a more detailed block diagram of the current control systemshown in FIG. 1;

FIG. 4 is a block diagram showing a portion of the drive control systemshown in FIG. 1;

FIG. 5 is a block diagram of the welding power supply system showingmeans for generating the welding current and protecting the torch andwork, both during arc initiation and welding operations;

FIG. 6 is a schematic diagram, partially in block form, of the circuitswhich control the initiation of the arc;

FIG. 7 is a block diagram of the vertical oscillation control systemschematically showing the circuit for controlling the verticaloscillation drive motor;

FIG. 8 is a block diagram of a portion of another system for controllingthe vertical oscillation motor;

FIg. 9 is a schematic diagram, partially in block form, showing some ofthe circuits depicted in FIG. 7 in greater detail;

FIG. 10 is a graph showing a relationship between current and arc lengthwhich is obtained by means of the system shown in FIGS. 7-9;

FIg. 11 is a graph showing the variation of arc current and arc lengthwith time;

FIG. 12 are waveforms illustrating the interrelation of variation of arcvoltage, arc length, and arc current with time;

FIG. 13 is an elevational view of the mechanism of the automatic arcwelder which embodies the invention;

FIg. 14 is the top view of the mechanism shown in FIG. 13;

FIG. 15 is a view similar to FIG. 13, but showing the filler wire feedmechanism is greater detail;

FIG. 16 is a plan view, partially in section, of a portion of themechanism shown in FIG. 13 which illustrates the lateral oscillatordrive mechanism in detail;

FIG. 17 is a fragmentary sectional view similar to FIG. 16, but showinga portion of the mechanism depicted therein in greater detail;

FIg. 18 is an elevational view, partially in section of the lateraloscillator mechanism shown in FIG. 16, but in a different position fromthat shown in FIG. 16;

Fig. 19 is a sectional view taken along the line 19--19 in FIG. 18; and

FIG. 20 is a perspective view of one of the work supports which is shownin FIG. 13.

Referring more particularly to FIG. 1, the arc welding torch 10 and thework 12 to be welded at a joint 14 are schematically depicted. The work12 may be sections of pipe, as shown in FIG. 13. FIGS. 13-20 alsoillustrate the mechanisms 16 which support and drive the torch 10 andthe work 12 so as to accomplish welding automatically. The electricalenergy for providing the arc which causes melting in the joint regionand of any filler material supplied thereto is provided by a weldingpower supply 18, the output terminals of which are connectedrespectively to the work 12 and to the electrode in the torch 10. Thetorch 10 also receives inert gas and cooling water from a water and gascontrol system 20. This system contains valving, as well as detectorsfor monitoring the flow of water and inert gas.

The driving mechanism 16, the welding power supply 18 and the water andgas control system 20 are all controlled so as to make the weldsautomatically. Automatic control is obtained by means of a timing andprogramming system 22 to be described in greater detail in connectionwith FIG. 2. When the welding cycle is initiated, say by pressing astart button, a command level is applied to the electrically controlledwater and gas valves in the water and gas control system 20. After anappropriate delay, as is obtained by digital delay devices in theprogramming system 22, a command is provided via a current controlsystem 24 to the welding power supply which energizes theelectromagnetically operated switches which connect the power lines tothe welding power supply 18. Digitally operative timing circuits in theprogramming system 22 are operative to provide an enabling level tologic circuits in a vertical oscillation control system 26 which iscoupled to the section of the drive mechanism 16 which enables verticaloscillation of the torch 10 with respect to the work, and thereforecontrols the height of the torch over the work 12.

After the height control command is provided by the programming system22 to the vertical oscillation control system 26, the circuits thereinwhich control the movement of the torch towards and away from the workupon starting or arc initiation are activated. After the arc isinitiated, other portions of the vertical oscillation control system 26are operative to sense the pulsating current which is supplied to thetorch by the welding power supply 18 under the control of the currentcontrol system 24, whereby to oscillate the torch vertically during thewelding cycle in synchronism with the current pulses. Digital timingcircuits in the timing programming system 22 also enable the drivecontrol systems 28 which control the lateral oscillation drive in thedrive mechanism 16, as well as the travel and wire feed drives of themechanism 16. Upon receipt of these command signals, the drive controlsystem 28 is then enabled to provide the preselected rates of lateraloscillation, travel and wire feed during the welding cycle. The drivecontrol systems 28 will be discussed hereinafter in connection with FIG.4.

The timing-programming system 22 also serves to shut down the system byproviding a command to the current control system to decrease thewelding current (hereinafter referred to as a slope enable command).This command is also used in the timing and programming system 22 togenerate commands for the drive control system 28 to stop the lateraloscillations and wire feed, as well as to control the verticaloscillation control system 26 to stop the height changes of the torchthus keeping the arc length constant. An output 30 which is obtainedfrom the current control system circuits, which produce the decreasingwelding current, is detected in the timing and programming system 22.When this output reaches a predetermined level, indicative of reductionof welding current to a level which will permit cut-off of such currentwithout difficulty due to transients and the like, the timing systeminitiates a clear command which stops all of the drives, shuts down thewelding power supply, as well as the supply of water and gas. The gasand water may be permitted to flow during a short post flow time aftershutdown by virtue of a delay circuit which is switched into the waterand gas flow control system 20 upon initiation of the clear command.

FIG. 2 is a simplified block diagram of the timing programming system22. The primary timing signatls are obtained from a clock source 32which may be the 60 Hz alternating current line. The clock source 32 mayinclude shaping circuits which provide pulses at the 60 Hz rate. Thesepulses are applied to a counter 34 in which the pulse rate is divided bysix to provide pulses at a 10 Hz rate. The 10 Hz pulses are then appliedto a chain of BCD counters 36. The outputs of the counters are convertedfrom binary coded decimal to decimal form in binary coded decimal (BCD)to decimal converters 38, one of which may be provided for each of theBCD counters. The conversion therefore produces a plurality of groups ofoutputs (5 being shown in the drawings) from the BCD to demimalconverters 38. The first group contains ten outputs, each correspondingto 1/10 second intervals. In other words, a pulse will occur on each ofthe outputs every 1/10 second, 2/10 second . . . to 9/10 second. Thenext group of outputs will provide pulses which occur at one secondintervals. Only one output is needed for pulses which occur every 1,000seconds.

Manually controllable switches (MSS) 40, 42 and 44 are used to selectgroups of combinations of outputs from the BCD to decimal converters 38.The selected outputs are applied to AND gates 46, 48 and 50 to producetiming signals which occur at selected intervals. The output of the ANDgate 46 connected to the first MSS 40 will be a pulse occurring at any1/10 second interval from 1/10 second to 99.9 seconds which is selectedby the MSS 40. Similarly, the outputs of the AND gates 48 and 50 whichare connected to the MSS 42 and 44 may be a pulse occurring at any 1/10second interval from 1/10 second to 1999.9 seconds. These pulses areapplied to and stored in flip-flops 52, 54 and 56. When the flip-flop 52is set, an enable travel command is applied to the drive control system28 (FIG. 1) so as to start the motor which drives the work 12.Similarly, after the time delay selected by the MSS 42, the flip-flop 54will be set and will apply an enable command to the current controlsystem 24 which will cause the system to apply a control signal to thewelding power supply 18 so as to change the magnitude of the weldingcurrent. This change in welding current magnitude is referred to hereinas a "current step." At the end of the welding cycle as preset by theMSS 44, the flip-flop 56 will be set and enable a slope command. Thisslope command is applied to the current control system and causes thewelding currents to be decreased gradually until the arc isextinguished. The enable slope command is also applied to a NAND gate58, together with the output of the flip-flop 52 which is complementaryto the output which is inverted in an inverter 60. By virtue of thelogical function of the NAND gate and the inversion of its output, theenable height lateral oscillation and wire feed command will be producedonly when the flip-flop 52 is set and the enable slope command is notpresent. The presence of the enable slope command inhibits the enableheight, lateral oscillator and wire feed commands, thereby stopping thewire feed and lateral and vertical oscillation of the torch and movingit away from the work. This is accomplished by means of logic circuitsin the welding power supply and vertical oscillator control system to bedescribed in connection with FIGS. 5 and 6. A slope level detector (notshown) in the current control system 24 generates a clear pulse when thecontrol current generated in response to a slope command decreases tothe level requisite for system shutdown. This pulse resets theflip-flops 52, 54, 56 and the counters 34, 36.

Other outputs (not shown) may be taken from the BCD to decimalconverters 38 via additional manual selection switches (also not shown)to control other functions in the automatic welder, such as the durationof the starting current which is produced by the current control system24, and gas pre-flow and post-flow before and after the welding cycle;the latter being obtained by commands applied to the water and gascontrol system 20 (FIG. 1).

The current control system 24 is schematically depicted in thesimplified block diagram of FIG. 3. The purpose of the current controlsystem is to generate a control voltage for the welding power supply 18.During normal welding operation, this control voltage is in the form ofpulses which are generated by a pulse generator 62 which may becontrolled to produce the pulses with different selected rates anddurations. A typical pulse rate may be of the order of 21/2 Hz. Atypical pulse duration may be 100 milliseconds during which the pulse ison or of high level, and 300 milliseconds during which the pulse is offor of low level. The pulse generator 62 itself may comprise a pair ofone-shot multivibrators, each of which has an adjustable time constantso as to select the pulse durations and rate. The one-shotmultivibrators are connected in a loop circuit so as to be free running.In the event that a constant high level or low level control votage isdesired to be supplied to the welding power supply so as to obtain aconstant current arc, rather than a pulsed arc, a control circuit may beassociated with the pulse generator to inhibit free running of the pulsegenerator and to condition the generator output to remain either at highor at low level.

The generator output signal is applied to the current amplitudeselection logic network 64 consisting of a suitable combination ofdigital gates. Also applied to this logic network 64 is the enablecurrent step command from the flip-flop 54 in the timing programmingsystem 22 (FIG. 2). The current amplitude selection logic networkprovides outputs selectively to three different manually controllablepotentiometers (MCP) 66, 68 and 70, depending upon the absence orpresence of a high level or low level pulse signal and the absence andpresence of the enable current step command. In the absence of theenable current command, the selection logic network 64 applies low levelpulses to the MCP 70 and the high level pulses to the MCP 68. Thepresence of the enable current step command and the high level pulsesignal provides an output to the MCP 66. In other words, the selectionlogic performs an exclusive OR operation in selecting the MCPs 68 or 70which independently vary or set the amplitudes of the level portions ofeach pulse cycle. The output of the MCP 68 is therefore labeled "highset," while the output of the MCP 70 is labeled "low set." The output ofthe MCP 66 is labeled high step since it provides an independent currentlevel which may be higher or lower than the selected high set level. Theactual amplitudes of the low set, high set and high step signals mayeach be independently set on their respective MCPs 70, 68 and 66 priorto welding operations to obtain the necessary current pulsecharacteristics.

In order to generate the requisite control voltage during arcinitiation, an amplifier (not shown) which produces a preset outputlevel when an enable arc initiation command is applied thereto may bealso provided in the current control system. Circuits in the timing andprogramming system provide the enable arc initiation command a presettime after the start switch is operated to allow time for gas and waterflow to be established.

The output of the last-mentioned amplifier, together with the outputs ofthe MCPs 66, 68 and 70 are applied to a summing network 72, which may bea resistor matrix, in order to develop the current control signal. Thiscontrol signal is amplified in a DC amplifier 74 and supplied as acontrol voltage to the welding power supply.

The summing network 72 also receives an input signal from a slopeamplifier 76. The slope amplifier is an integrating amplifier andgenerates a slope signal by charging the capacitor in its feedbackcircuit, or charging circuit 78. An MCP 80 is connected to the chargingcircuit 78 so as to control the charge time constant and therefore rateof change of the slope voltage which is produced by the slope amplifier76. A trigger circuit 82, such as an FET (field effect transistor) gateis turned off to allow the charging circuit 78 to charge upon thereceipt of a slope initiate command. The flow detection and safetycircuits can also command a slope.

It will be recalled that the slope initiate command is provided at theend of a normal welding cycle. Slope control voltages may, however, begenerated under other circumstances. For example, in the event thatthere is a failure in the gas or water supply system 20 (FIG. 1),detector circuits in that system provide outputs which operate a safetytrigger circuit 86. This circuit will also cause a slope initiatecommand and may include additional resistors so as to insure thenecessary rate of change (slope-out) of the welding current.

The drive control system 28 (FIG. 1) may include a motor control system,such as shown in FIG. 4 for the travel motor which drives the work, aswell as for the wire feed and lateral oscillation drive motors. Each ofthese drive systems includes a DC servo motor, such as the motor 90,shown in FIG. 4. A tachometer 92, which is coupled to the shaft of themotor, provides a feed back signal to a servo amplifier 94 which may bea direct current amplifier which provides an amplitude controlled DCdrive signal to the motor 90. The amplitude of this drive signal fromthe output of the servo amplifier is controlled by an MCP 96. A runcommand which is associated with the motor 90 is applied to the input ofthe MCP 96. Thus, when the run command is not present, the servoamplifier will produce no operating current for the motor 90 and themotor 90 will stop. The presence of a run command therefore enables theoperation of the drive mechanism and the absence of the command inhibitssuch operation.

The welding power supply 18 is shown in FIG. 5. It includes a threephase alternating current supply 100 provided by a three phasetransformer which is connected to the line via the electromagneticallyoperated actuators which pull in when the start button is pressed anddrop-out at the end of the welding cycle, as was explained in connectionwith FIG. 1. The AC supply 100 feeds a three phase magnetic amplifier102 having bias, control and output windings. DC bias for the biaswinding is obtained from a bias supply rectifier circuit 104 coupled tothe AC supply. The terminals 106 and 108 are connected to opposite endsof the control windings. The output windings are connected to rectifiers110 which provide the welding current across negatively and positivelypolarized output terminals. The negative terminal is connected to thetorch electrode, while the positive terminal is connected, after passingthrough the primary winding of a rate feedback transformer 111, to thework which is desirably grounded.

Current feedback is obtained from current transformers 112 which areinductively coupled to each of the three phase lines from the magneticamplifier 100 to the rectifiers 110. The current transformers outputsare rectified in diode bridge rectifying circuits for example, whichconstitutes the rectifier 114. The rectified output voltage is passedthrough a low pass filter 116 to remove the ripple component thereof,and is applied through a summing circuit 118 which may be a resistivematrix, together with the control voltage from the output amplifier 74of the current control system (FIG. 3). The DC current feedback from therectifiers 114 is opposite in polarity to the control voltage so as tostabilize the welding current which is produced to the current which ispreset or ordered by the control voltage. The output of the summingcircuit is connected to a DC amplifier 120. The output of the amplifieris connected across the magnetic amplifier control winding (viz.terminals 106 and 108) in series with the secondary of the rate feedbacktransformer 111 which applies a predetermined level of the ripplevoltage in opposite phase to effect the cancellation thereof. It also isfed to a monitor 121 which determines if it is within a predeterminedacceptable range. A Current OK signal is the output.

Inasmuch as the control voltage will normally be a pulse train, thewelding current will consequently also normally be in the form ofpulses. The DC amplitude of the current, however, may readily bedetected by a weld current amplitude detector 122. This detector 122provides separate outputs to a protective switch circuit which providesa short circuit between the torch electrode and the work during startingof the arc in order to protect both the electrode and the work. Theother output from the amplitude detector is applied to a thresholdcircuit 126 which enables the height control (viz. the verticaloscillation of the torch) when the welding current is greater than apredetermined magnitude, say 27 amperes.

The protective switch circuit 124, the amplitude detector 122 and thethreshold circuit 126 are shown in detail in FIG. 6.

The starting protective switch circuit 124 prevents current flow fromthe torch electrodes to the work until the current transient disappearsand conditions are right to form an arc. This circuit, as shown indetail in FIG. 6, contains a main silicon control rectifier (SCR) 130which is connected in series with a fuse across the welding currentsupply line 132 from the rectifier to the torch electrode. When thewelding supply is turned on, the voltage between the line and ground issubstantially full magnitude, typically 80 volts. The voltage charges apair of capacitors 134 and 136. The capacitor 134 has a resistor 141connected thereacross in order to prevent retriggering of the SCRs inthe circuit 124, as will be explained more fully hreinafter. Thesecapacitors 134 and 136 are permitted to charge only during starting byreason of the light dependent resistor 138 which is connected in theircharging current paths. This resistor is illuminated by a lamp 140 onlyat such time as the current being supplied by the welding power supplyis low, say below 6 amperes. This current is detected by means of athreshold amplifier 142 which is connected to the rectifiers 114 in thewelding power supply (see FIG. 5). So long as the threshold is notexceeded, a transistor 144 is operated to its conductive state. When thelamp 140 goes off, as occurs when the welding current exceeds the 6amperes level, the resistance presented by the light dependent resistor138 is high and effectively presents an open circuit. The slight voltagedrop across the resistor 141 is too low to permit the capacitor 134 tocharge to a level sufficient to generate a triggering voltage for theSCR 130.

During starting, however, the capacitor 134 soon attains its full chargewhich is sufficient to obtain breakdown in a silicon controlled switch146. This switch is also known as a "Diac"; Diac being a trade name ofthe General Electric Company. Current from the discharge of thecapacitor 134 therefore flows through the Diac, experiencing a smallvoltage drop therein, and thence through a diode 148 and the primary ofa transformer 150. The pulse which is generated upon breakdown of theDiac is translated through the transformer 150 to the trigger electrodeof the main SCR 130, causing it to break down. Thus, a short circuit isestablished. The capacitor 136 cannot discharge through the circuitcontainin g the Diac 146, diode 148 and transformer 150 in view of thepolarization of a diode 152 which is in series with the capacitor 136 inthis circuit.

When the current to the short circuit (SCR 130) reaches 6 amperes,approximately, the signal from the current transformer rectifier 114(FIG. 5) exceeds the threshold set in the amplifier 142. Thus, the lamp140 is extinguished and the resistance of the light dependent resistor138 increases to its open circuit value. The weld current amplitudedetector 122 is operative to cause the breakdown of a pair of secondarySCRs 154 and 156, thereby providing a discharge path for the capacitor136. This discharge path is completed through the conductive primary SCR130. However, the current flow from the capacitor 136 is in a directionopposite to the current flow between the line 132 and ground. In otherwords, a bucking voltage to the voltage to which the capacitor 136 ischarged opposes the voltage tending to sustain the breakdown of theprimary SCR 130. The SCR 130 is thereupon cut off removing the shortcircuit between the torch electrode and the work. Upon the discharge ofthe capacitor 136, voltage which sustains the conduction through thesecondary SCRs 154 and 156 is also no longer present, thereby causingthese SCRs to become nonconductive.

After a predetermined preflow time, and if the current and voltage ofthe welding power supply output are normal the height control isenergized and the torch drives to touch the work. The current now beginsto flow through a saturable transformer 158.

The weld current amplitude detector 122 contains saturable transformer158 which controls the current flowing around a circuit including thesecondary of a transformer 160 and a resistor 162. The transformer 160is connected across the alternating current supply line. Therefore, asthe saturable transformer 158 becomes partially saturated due to thetorch touching the work, the voltage drop thereacross decreases the thevoltage appearing across the resistor 162 increases. The voltage acrossthe resistor 162 is rectified by a half wave rectifier circuit includinga diode 164 and a capacitor 166. A resistor 168 completes the rectifiercircuit. The voltage developed across the capacitor 166 in the rectifiercircuit is sufficient to break down a Diac 170 when it reaches a valuecorresponding to a current flow through the line 132, equal toapproximately 6 amperes. Breakdown of the Diac sends a current pulsethrough the primary of a transformer 172, the secondary windings ofwhich are connected to the trigger electrodes of the secondary SCRs 154and 156. Thus, these SCRs then fire to remove the short circuit.

The weld current amplitude detector also includes another half waverectifier circuit 174 which is connected across the resistor 162. Thevoltage developed across this amplitude detector appears across apotentiometer 176 which is part of the threshold circuit 126. Thethreshold circuit itself includes a zener diode 178 which is connectedthrough a coupling resistor 180 to the base of a transistor 182. Whenthe threshold is exceeded, the zener diode 178 conducts, causing thetransistor 182 to conduct. The voltage level at the collector of thetransistor 182 is used to trigger a JK flip-flop 186 which enables thevertical oscillation drive or height control mechanism to pull the torchaway from the work, thereby starting the arc. To this end, the thresholdat which the transistor 182 becomes conductive may be adjusted so thatthe proper starting current (e.g. 27 amperes) is at that time flowingthrough the torch. The threshold circuit 126 also actually measures thecurrent to the torch by virtue of being connected to the weld currentamplitude detector 122. Thus, there is reasonable assurance that theproper welding current is flowing through the torch at the instant whenstarting is desired. Starting, as well as vertical oscillation, is underthe control of the programming system and cannot occur until the enableheight command from the inverter 60 (FIG. 2). Flip-flop 186 can betoggled if the J input is high. The J input is high if the torch currentis within normal limits. This is to prevent possibly burning a hole ifthe current is very high, as the torch lifts from the work. If thecurrent in the torch is normal, a ground excites lamp 188, thus changingthe light dependent resistor 188 to its minimum value. This lowresistance now makes the J input high. The necessary voltage levels aredetermined by the voltage division accomplished by resistor 190 andlight dependent resistor 188. The clear input to the flip-flop is low toclear the flip-flop and high to enable. When the electromechanicalactuator is actuated, the level to the flip-flop 186 is high. When theactuator is de-energized the level is low.

The vertical oscillation control system is illustrated in greater detailin FIG. 7. It includes a source of direct current operating voltageindicated as being a DC supply 200. The amplitude of the oscillation iscontrollable by adjusting the positive value of the voltage which isapplied to a summing circuit 202 which may be a resistor matrix. Thiscontrol over this voltage is obtained by a manually controllablepotentiometer 204 which may have separate sections so as to vary thisvoltage in coarse and fine steps. The voltage between the torch and thework (viz. the arc voltage) is sensed by an arc voltage sensing circuit206. The polarity of this voltage is opposite to the polarity of thevoltage applied from the supply to the summing circuit 202 via the MCP204. Overdrive circuits 208 and 210 which provide voltages of oppositepolarity to drive the torch, respectively, out or away from the work,and in or toward the work are also provided, principally for startingpurposes. Another circuit is provided indicated as being a corona delaycircuit 212. This circuit applies an overriding voltage to the summingcircuit during the short time interval immediately after a voltage dropappears between the torch and the work, until the effective coronaresulting from that drop have time to dissipate. The output of thesumming circuit controls a DC servo amplifier 214.

The summing circuit output to the amplifier may be considered as acontrol voltage which varies in amplitude and sense in accordance withthe direction in which it is desired to move the torch. When the voltageis zero, no torch movement is, of course, desired. When the voltageincreases in one sense, say positive, it is applied to a diode 216 totrigger a solid state switch 218 of the type known as a "Triac." Triacis a trademark of the General Electric Company for such solid stateswitches. Similarly, if the error voltage amplified by the DC amplifier214 is of the opposite or negative polarity, current correspondingthereto passes through an oppositely polarized diode 220 and triggersanother Triac 222. The Triacs switch the alternating current flow from atransformer 224, which is connected across the alternating current line,through either of the split phase windings 226 and 228 of the servomotor 230. The shaft of the motor therefore may turn in oppositedirections, causing the vertical oscillation drive mechanism which iscoupled thereto to oscillate. This oscillation is principally a functionof the pulsating arc current as sensed by the arc voltage sensingcircuit 206.

The curves shown in FIGS. 10, 11 and 12 illustrate this variation in arccurrent (I_(w)) and the consequent variation of the height (H) of thetorch over the work.

FIG. 10 illustrates the variation of height with arc current whichoccurs when the arc voltage is maintained constant, as is accomplishedby means of the servo system shown in FIG. 7. As the arc currentincreases, the arc voltage tends to follow this increase. Thus thevariation in the height of the torch by reason of the control systemshown in FIG. 7 tends to decrease the height (viz. the height is aninverse function of the current). This relationship is shown in FIG. 11.The pulsating arc current which is produced by the current controlsystem therefore tends to produce with a slight delay a pulsating arcvoltage, as shown in FIG. 12. The oscillations in the height of thetorch with respect to the work follow this arc voltage as shown in FIG.12. A uniform oscillation therefore is produced by reason of thevertical oscillation control mechanism and circuits because the currentcontrol system produces current pulses which force such oscillation.Without such current pulses, oscillation would not be produced. Suchoscillation is an important parameter in controlling the formation ofthe weld. The system provided by the invention and the method of theinvention is to produce such oscillations and then control them toobtain the desired weld characteristics.

In the alternative embodiment of the verical oscillator control system,shown in FIG. 8, the output of the summing circuit 202 is amplified inan amplifier 235 which is similar to the amplifier 214. The output ofthis amplifier 234 is divided into two channels, one of which contains asingle amplifier stage 236, while the other contains a pair of stages238 and 240 connected in tandem. Thus, the output of the amplifier 236will be 180° out of phase with respect to the output of the amplifier240. These signals are applied as gating signals to pulse transformers242 and 244. The pulse transformers gate signals from an oscillator 246which provides a constant frequency output signals, which is desirablyin the range of 600 Hz to 2 KHz, to the motor control system. This motorcontrol system may contain Triacs, such as the Triacs 218 and 222 whichare separately triggered by the outputs of the pulse transformers,depending upon which of the transformers receives a control signal ofproper polarity to enable the transmission of the oscillationtherethrough. The distance which the torch moves away and then towardsthe work depends, of course, upon the duration of the control signalwhich is applied to the pulse transformers 242 and 244. Inasmuch as thefrequency of the oscillator 246 is relatively high, the movement of thetorch in any direction occupies a number of cycles of the oscillatorfrequency and threfore is essentially continuous.

The summing circuit 202 is shown in FIG. 9 as comprising a matrix ofresistors 250, 252, 254, 256 and 258 which are connected to a commonnode or summing point 260. The torch itself is connected between theterminals 262 and 264, with the electrode of the torch (which ismaintained at a negative potential) connected to the terminal 262, whilethe work or ground terminal corresponds to the terminals 264.

The resistor 266 and capacitor 268 connected between the torch and itssumming resistor 250 provide filtering of the power line frequencycomponent which appear at the torch electrode. A resistor-capacitornetwork 270 is connected across the output of the rectifiers 114 andprovides the adjustable signal to the summing point 260. This adjustablesignal may be varied from zero to a maximum level by potentiometer 274.With potentiometer 274 at zero, the vertical oscillation is maximum andinversely proportional to arc current. Conversely, with potentiometer274 at a maximum the vertical oscillation is maximum, but the motion isnow in phase with the arc current, i.e., when the current increases thearc length increases. Another feature of this control is that whenpotentiometer 274 is at its center position, the vertical oscillationcan be completely cancelled, and pure arc voltage control is obtained.

The voltage appearing across the torch terminals 262 and 264 isamplified in a threshold amplifier 278 which provides an output forenabling an AND gate 280 when the arc voltage is above a certain levelindicating that an arc can be maintained or is already present. The ANDgate, however, is not conditioned to provide the desired logical outputlevel, unless and until a weld current present command (viz. 6 ampereswelding current) is obtained from the flip-flop 186 (FIG. 6) which istriggered when the current control voltage and proper welding currentamplitude is present and the programming system has also provided acommand to enable height control. With the logical command present atthe input to the AND gate 280, an output is provided which inhibits alamp driving amplifier 282 which provides enabling current to a lamp L2linked to a light dependent resistor L₂ DR which is connected between asource of negative potential indicated as -B and the summing matrixresistor 256. Accordingly, the lamp L2 is normally illuminated, therebylowering the resistance of the resistor L₂ DR so that a negativepotential may be applied to the summing point 260. This negativepotential provides a "drive-in" command and so long as it is maintainedkeeps the torch in contact with the work. Sufficient resistance ispresent in the electrode material of the torch to provide the voltagelevel necessary to operate the threshold amplifier 278. It will berecalled also, that the SCR 130 (FIG. 6) provides a short circuit acrossthe terminals 262 and 264 until the welding current is sufficient tostart and maintain the arc. This provides an additional safety featurewhich militates against improper starting which could cause incompletemelting or sputtering with consequent damage to the work. As soon as thearc current is present and the other conditions for arc starting andmaintenance of vertical oscillation exist, the lamp L2 is extinguished,thereby preventing the application of the negative voltage from thesource at -B. A positive voltage is, however, applied to the summingpoint from the MCP 204 (FIG. 7) thereby permitting the torch to moveaway from the work so as to start the arc and maintain it under thecontrol of the vertical oscillation control system such that the torchwill execute the vertical oscillations in response to the pulsatingcontrol current, as was explained in connection with FIGS. 10-12.

In order to preclude starting until the dissipation of corona effects,the corona delay circuit 212 includes a capacitor 286 which is chargedthrough the transistor 288 when the light dependent resistor L₂ DR isilluminated. When L₂ DR is extinguished, the potential maintained by apair of zener diodes 290, which had previously biased the transistor 288to conduction, is cut off. The capacitor 286 then discharges through thesumming network resistor 258 and applies a positive potential to thesumming point 260 for a time sufficient to preclude downward movement ofthe torch toward the work until the corona effects dissipate.Accordingly, vertical oscillation is inhibited for a short period oftime, thereby preventing possible damage to the work due to coronaeffects.

A zener diode 292 which is connected in series with a resistor 294 and alamp L₁ linked to the light dependent resistor L₁ DR across the torchand work electrodes 262 and 264 provide a safety threshold so as todrive or maintain the torch away from the work if the weld voltageexceeds a certain safety threshold. The light dependent resistor L₁ DRis therefore normally extinguished until the safety threshold isexceeded. Thereupon, a positive voltage is applied via the summingmatrix resistor 254 to the summing point which drives or maintains thetorch in an upward position away from the work.

It will therefore be seen that the system of circuits so far describedprovides the control signals which automatically drive the torch withrespect to the work. The mechanisms which are responsive to the controlsignals operated by the motors which are described in connection withthe vertical oscillation and drive control systems are shown in FIGS.13-20 of the drawings.

Referring first to FIGS. 13 and 14, the work to be welded is shown as asection of pipe 300 which may have a large diameter, say of the order of15 inches. A carriage 302 is clamped to the pipe 300 by means of feet304 which engage the outer diameter of the pipe at four diametricallyopposed points. These feet 304 are shown in greater detail in FIG. 20.The carriage portion on which the feet 304 are mounted is made up of twosemi-circular sections 306 and 308. These sections are pivotable awayfrom each other about a pivot pin 310. The junction 312 of the sections306 and 308 which come together when the carriage is clamped on the pipe300 is provided with latches 313 and a clamp 315 which hold the sections306 and 308 in clamped relation on the pipe.

The feet 304, as shown in FIG. 20, is made up of a stanchion 314 whichis screwed into the carriage 302. The upper portion of the stanchion hasa slot 316 therein in which a flat spring 318 is secured by means of ascrew 320. The spring 318 is mounted on a block spacer to provideclearance. A pair of buttons 322 actually engage the pipe. When thecarriage sections 306 and 308 are closed, the spring plates locatethemselves into the slots 316 and the proper amount of tension issupplied to the pipe to hold the carriage firmly in position withoutdamage to the exterior walls of the pipe and to take care of heatexpansion of the pipe.

The welding torch 10 and the portion of its drive mechanism 16 whichprovides for vertical and lateral oscillation thereof in a directiontowards and away from the pipe and also in a direction along the axis ofthe pipe, are mounted on a yoke 324 which in turn is mounted on a flangeportion 326 of a split worm gear which is located in a slot in thecarriage 302 and rotates around the pipe 300 as the worm gear is driven.A travel motor 328 and its associated worm gear mechanism 330 is securedto the carriage 302 by means of bolts 332 which extend through holes inthe mechanism 330. Thumb screws 334 hold the worm gear mechanism 330 andits associated motor 328 so tha the worm in the mechanism 330 engagesthe worm gear part of the flange 326. Accordingly, the worm may bedisconnected from the worm gear so as to facilitate the adjustment ofthe angular position of the pipe 300 with respect to the carriage 302.The split gear carriage drive mechanism consisting of the worm 330,flanged worm gear and travel motor 320, may be of a construction similarto that described in U.S. Pat. No. 3,389,846, issued June 25, 1968.Reference may be had to that patent for a detailed description andshowing of a suitable carriage drive mechanism.

In order to facilitate the fastening of the yoke to the flange 326, theedge 336 of the yoke 324 is beveled and is inserted into a complementarybeveled slot in the periphery of the flange 326. A pair of set screws338 are located near the ends of the yoke and hold the yoke in positionin the flange 326. In addition, another set screw 340 extends throughthe end of the flange 326 and through the yoke's tapered edge 336 tofirmly hold the yoke and the drive mechanism 16 mounted thereon inposition for rotation about the pipe 300.

The principal parts of the drive mechanism 16 which are mounted on theyoke 324 are the vertical oscillation mechanism 342, the lateraloscillation mechanism 344 and the wire feed mechanism 345. The wire feedmechanism 345 is shown in greater detail in FIG. 15. It includes a drivemotor 346 which drives a pair of pinch rollers between which filler wire348 is pulled from a reel 350 and driven through a tubular guide tip352. The end 354 of the tip 352 is flexible. Accordingly, when the torch10 is lowered to the pipe 300 during starting of the arc, the torchhousing engages the tip and bends it out of the way so that there is nointerference or inadvertent contact of the torch electrode with the wire346. When the torch is raised to normal position, the tip returns to theproper position for feeding wire into the joint being welded.

The entire drive mechanism 16 is fastened to the yoke 324 by means ofscrews (not shown) which extend laterally through the yoke. The verticaloscillation mechanism 342 includes a drive motor 356 which is fastenedto a platform 358 which is oscillated laterally by the lateral drivemechanism 344. Coupling between the platform 358 and the lateral drivemechanism 344 is accomplished by means of a pin 360, fixed to theplatform 358, which extends downwardly from the platform 358 intoengagement with a laterally oscillatory slotted member 362 which formspart of the lateral oscillating mechanism 344. The pin and slottedcoupling member 362 are shown in FIGS. 16 and 17 and will be discussedin greater detail in connection with these figures. The lateraloscillatory mechanism itself includes a drive motor 364 and a motiontranslating mechanism 366 which converts the rotary motion of the motor364 into controllable amplitude lateral oscillations so as to drive thepin 360 and the platform 358.

The shaft 368 of the motor 356 has a gear at the end thereof whichdrives a spur gear 370. The spur gear 370 drives a pinion (not shown)mounted on the same shaft as the gear 370. This pinion drives a rackgear 372 which is connected to a slide 374 which is connected to abracket 376 by means of a dovetail coupling 378 which slides in a slot380 in the bracket 376. Thus, as the motor rotates, the slide 374 willmove up or down, depending upon the sense of motor rotation. Theposition of the slide 374 may be adjusted manually by means of a knob382 which is carried with the slide 374. The end of the knob whichextends through the slide 374 has a gear 384 which is easily rotated byrotating the knob 382. This gear engages the rack gear 372. Thus, byturning the knob 382, the position of the slide may be adjusted. Aworm-worm gear drive may be used to drive the slide 374 up and down. Arack gear arrangement, as shown, is however preferable since its partsare lower cost and reliable.

A bracket 386 is attached to the slide by means of a hold down knob 388which is carried on a bolt extending from the slide 374 through a slot390 in the bracket 386. The torch 10 is carried on an L-shaped bracket394 which is also secured in the bracket 386 which moves with the slide374. The wire feed mechanism 345 is also fastened to the bracket 386.

Reference may be had to FIGS. 16 and 17 for a view of the platform 358and the translation mechanism 366 for laterally oscillating the platform358. The platform 358 is slideably mounted between two side plates 396and 398. The side plate 396 is also shown in FIGS. 13 and 14. The backside plate 398 is fastened to the yoke 324, as shown in FIG. 14. Theside plates are spaced from each other by means of posts 400. Bushings402 in the platform are provided so that the posts 400 guide themovement of the platform 358 from side to side between the side plates396 and 398. The platform 358 may be biased by means of a spring 404towards the side plate 396. This spring is designed to counteract theweight of the torch, vertical oscillation motor and other equipmentsupported by the platform 358 should the mechanism be positioned suchthat the platform 358 must move up and down rather than horizontally asshown in the drawings.

The translation mechanism 366 is coupled to the platform 358 by means ofa slotted bar 406 which is laterally oscillated by the mechanism 366.This bar has a bifurcated end 408 through which extends a threaded pin410 on which the slotted coupling memeber 362 is located. This pincarries threads 411 engaging the threaded coupling member 362. Byrotating a knob 415 (see also FIG. 1), threaded coupling 362 is carriedby the threaded pin 410 as it rotates, thereby adjusting the position ofthe threaded coupling member 362.

The pin 360 which depends from the platform 358 is located in the slotin the coupling member 362. Accordingly, by rotating the knob, 415, thecenter of the oscillatory path of the platform 358 and therefore of thetorch can be set. This cross seam adjustment is especially desirable inthat it selects the portion of the joint over which the oscillationtakes place. As will be described hereinafter, the amplitude of theoscillation is adjustable by means of the translating mechanism 366.

The translating mechanism is shown in FIGS. 18 and 19 as well as in FIG.16. It will be recalled that the slotted bar 406 is coupled to thecoupling member 362 so that the lateral oscillation thereof can betranslated to the platform 358 via the pin 360 which depends from theplatform. The slotted bar 406 is slideably mounted on a pair of pins 414and is translated from left to right as shown in FIG. 16 by a mechanismof gears, cranks and pins 416 (FIG. 18). In this mechanism 416 the shaft418 of the motor 364 is attached to a crank 420, the arm 422 of whichrotates about the axis of the shaft 418 as the shaft 418 is rotated. Apinion 424 having a collar 426 is mounted on the arm 422 so that it mayrotate freely. The pinion 424 m shes with an internal gear 428 having agear ratio with respect to the pinion 424 of 2:1. This internal gear 428also has an external thread which meshes with a worm 430. The worm 430may be turned by means of the slotted head 432 thereof so as to rotatethe internal gear 428. The limits of such rotation are established bymeans of a pin 438 and a slot 436 in the internal gear 428. Anothercrank 440 is fastened to the collar 426 by means of a set screw 427 sothat the crank 440 rotates therewith. The arm of the crank 440 is in theform of a pin which fits into the slot 444 in the slotted bar 406.

By turning the internal gear 428 with the worm 430, the angle ofreciprocation of the crank 440 may be adjusted. Thus, when the motor 364rotates, the amplitude of lateral oscillation of the pin 442 may bedecreased from the maximum amplitude of oscillation thereof. Suchmaximum amplitude of oscillation is obtained when the internal gear isin the position shown in FIG. 19 of the drawings. This amplitude may bedecreased to zero by rotating the angle of reciprocation of crank 440 tovertical in FIG. 16.

While the gear mechanism 416 is preferred inasmuch as it provides veryfine adjustment of the amplitude of lateral oscillation of the torch 10,as well as the position of the torch, other mechanisms which affordadjustable lateral oscillation may also be provided. For example, a dischaving an end face disposed at an angle with respect to its axis may beused as a cam by being spring biased against a follower, such as a ballbearing which is disposed eccentrically with respect to the axis of thecam disc. A housing may be coupled to the cam disc which may be rotatedby suitable gearing, such as bevel gears which extend into the housing.By adjusting the eccentricity of the cam disc with respect to itsfollower, the disc and the housing will oscillate laterally at setamplitudes corresponding to the set eccentricity.

From the foregoing description, it will be apparent that there has beenprovided an improved fully automatic welding system which is capable ofpracticing the improved method of arc welding which utilizes currentpulses both to generate a pulse arc and to effect the oscillation of theheight of the torch with respect to the work. The method and system alsoaffords adjustable parameters of lateral oscillation and torch-to-worktravel along the weld, as well as of the weld current pulses themselves.While illustrative embodiments of the system have been described, itwill be appreciated that variations and modifications therein, withinthe scope of the invention, will undoubtedly suggest themselves to thoseskilled in the art. Accordingly, the foregoing description should betaken merely as illustrative and not in any limiting sense.

We claim:
 1. An automatic arc welding system comprising:a. a torch forproviding an arc with which work can be welded, b. means for drivingsaid torch along a vertical line between said work and said torch whoseposition is responsive to a control voltage applied thereto, c. meansfor providing to said torch direct current welding current having aperiodic waveform, d. means for sensing the current through said torchand generating a signal proportional thereto, e. means for sensing thevoltage across said torch and generating a signal proportional thereto,and f. means for controlling said torch driving means by subtractingsaid signal proportional to said current from said signal proportionalto said voltage, to produce said control voltage.
 2. An automatic arcwelding system as recited in claim 1, further comprising:g. means forvarying the amplitude of the signal proportional to said current.